Power control method and a radio system

ABSTRACT

The present invention relates to methods of controlling transmitting power in a cellular radio communication system comprising one or several base stations each communicating with mobile terminals located within its respective area of coverage in which method the desired power change is calculated by means of an adaptive algorithm. Power drift between the desired output power and actually used output power is reduced by making the algorithm reduce the influence of the previously used output power by multiplying it with a “forgetting function” λ with values between 0 and 1. At the receiving end is a decision made whether the received power order commands are reliable or not by checking the soft value of the power order commands. This proposed power control algorithm provides a delta-modulated fast power control method which is robust to errors in the signalling channel and at the same time reacts quickly to fast fading variations.

This application claims priority under 35 U.S.C. §§119 and/or 365 to9903364-9 filed in Sweden on Sep. 17, 1999; the entire content of whichis hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to methods of controllingtransmitting power in a cellular radio communication system. Moreparticularly the invention relates to a fast, robust and adaptive powercontrol method.

The invention also relates to an arrangement for carrying out themethod.

DESCRIPTION OF RELATED ART

The capacity of CDMA (Code Division Multiple Access) systems isinterference limited since the channels are neither separated infrequency nor separated in time. A single user exceeding the limit ontransmitted power could, in a surrounding area, inhibit thecommunication of all other users. Thus, power control is very importantin interference limited systems such as CDMA or wideband-CDMA.

The power control systems have to compensate not only for signal qualityvariations due to a varying distance between base station and mobileterminal, but must also attempt to compensate for signal qualityvariations typical of a wireless channel. These variations are due tothe changing propagation environment between the base station and themobile terminal as the mobile terminal moves across the cell or as someelements in the cell move. There are basically two types of channelvariations, slow fading and fast fading.

Several proposals have been made to mitigate these two types of fadingin CDMA systems. A well-known prior art power control method is thatwhich is found in interim standard 95 (IS-95) systems. The IS-95 reverselink power control mechanism consists of two parts, open loop powercontrol and closed loop power control. The open loop power control isused to adjust the mobile terminals' transmitting power based on thereceived power from the base station. Assuming that the radioenvironment is reciprocal in both forward and reverse link, the mobileterminal adjusts its reverse link transmission power according to thereceived power on the forward link, i.e. if the received power is forexample 5 dB lower than expected, the mobile terminal raises its outputpower with 5 dB which is a good estimate of the path loss on the reverselink.

However, because of the frequency separation between reverse and forwardlinks, the fast fading of the two links are independent. To account forthis difference and to further control the mobile terminals' transmitpower, closed loop power control is used. In the closed loop powercontrol mechanism, the base station demodulates the reverse link anddetermines the signal to noise ratio, SNR, of the user. If the SNR islower than a desired threshold the base station orders the mobileterminal to raise the transmit power. If the SNR is higher than adesired threshold the base station orders the mobile terminal to lowerthe transmit power.

A significant part of the closed loop power control mechanism is thespecification of a fixed power step size. Each power adjustment commandorders the mobile terminal to either raise or lower its output powerwith 1 dB. The 1 dB fixed change per power order command is chosen basedon a compromise of different radio environments, ranging from astationary mobile terminal to a high speed vehicle and of differentchannel types.

If the signal quality changes quickly it is necessary for the powercontrol mechanism to follow these variations, i.e. if the signal qualitygoes down a power command ordering the transmitter to raise the usedoutput power should be issued. The 1 dB fixed change per power ordercommand then causes problems, since, if the power step size is toolarge, i.e. the quantization is too small, unnecessary fluctuationsoccur around the desired power level causing unnecessary interference.If, on the other hand, the power step size is too small the powercontrol mechanism is not capable of following fast variations in signalquality, since the required number of necessary power order commandsgrows too large, i.e. the slew rate is not large enough.

In WO-9726716 is described a method of dynamically controlling the powerstep size in such a way that, on the basis of the power order commandsto be examined, a calculation is made of the number of two successivecommands in different directions in proportion to the total number ofcommands during a specific time period. The calculated proportion isthen compared to a reference value and the power step size is changedaccording to this comparison.

In WO-9851026 is described a method where the power step size is changedin discrete steps, e.g. 0.25, 0.50 and 1.00 dB based on systemconditions, such as the speed of the mobile terminal.

The general problem with the currently known approaches is that thepossibilities of adjusting the power step size are too coarse and thatthe methods are not robust enough against errors in the signallingchannel, which can cause power drift between the actually used outputpower and the desired output power.

SUMMARY OF THE INVENTION

The present invention deals with the problem of how to achieve anadaptive power control method that is robust against errors in thesignalling channel and at the same time reacts quickly to fast fadingvariations.

One way of realising a power control, which reacts quickly to fastfading variations, is to delta-modulate the desired power, that is,transmitting simple power commands on an in-band signalling channel thatorder the mobile terminal or the base station to either raise or lowerits respective output power. In order to avoid large control loop delaysthese power commands should be transmitted without channel encoding andoutside any interleaving. This implies that there will be bit errors inthe power commands. The delta-modulation must therefore be robust toerrors and at the same time able to react quickly to fading variations.

Thus, an object of the present invention is to realise a power controlmethod in such a way that no strong fluctuations occur around thedesired power level and that it is robust against errors and stillcapable of reacting quickly to, and following fast fading variations.

This object is achieved according to the invention by means of a fastpower control method, in which method the desired power change iscalculated by means of an adaptive algorithm, with channel quality andestimated previously used output power as input data. Power drift,caused by bit errors in the in-band signalling channel, between thedesired output power and the actually used output power is reduced bymultiplying the estimated previously used output power value with a“forgetting function”, with values between 0 and 1, reducing theinfluence of the estimated previously used power outputs. Finally, atthe receiving end the new output power from the transmitter iscalculated.

In a first embodiment of the invention the average power step size ismade dependent of the time correlation of channel quality. If thecorrelation of channel quality is high the power step size could belarger than if the correlation of channel quality is low, since thebehaviour of the channel then is more uncertain.

In a second embodiment of the invention the “forgetting function” λ ismade dependent on the time correlation of channel quality. If thecorrelation of channel quality is high the “forgetting function” λshould be large to give old values higher impact resulting in a fastersystem. If, on the other hand, the correlation of channel quality is lowthe “forgetting function” λ then should be made small to reduce theimpact of old power values since the channels behaviour is uncertain.

In a third embodiment of the invention the power step size is dependenton the soft value of the power order command. If the soft valueindicates a large probability for bit errors the power step size shouldbe small and if the soft value indicates a low probability for biterrors the power step size should be larger since the power order thenis more reliable.

In a fourth embodiment of the invention the “forgetting function” λ ismade dependent on the soft value of the power command. If the soft valueindicates a large probability for bit error the forgetting function λshould be made small and if the soft value indicates a low probabilityfor bit error the “forgetting function” λ should be made large.

In a fifth embodiment of the invention the power step size is madeindirectly dependent on channel quality by defining an area within whicha difference between the desired output power and a long term mean valueof the desired output power is allowed to change.

By using the proposed power control method a fast power control isachieved with long-term robustness and good adaptability to thevariations in the in-band signalling channel.

The term “comprises/comprising” when used in this specification is takento specify the presence of stated features, integers, steps orcomponents but does not preclude the presence or addition of one or moreother features, integers, steps, components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a radio system to which the method of the invention can beapplied.

FIG. 2 shows a flowchart describing the power control method.

FIG. 3 shows a summary of the different calculations in the method.

FIG. 4 illustrates the relationship between the power step size Δp andthe time correlation of channel quality.

FIG. 5 illustrates the relationship between the power step size Δp andmeasurement errors in the channel quality.

FIG. 6 illustrates the relationship between the “forgetting function” λand the time correlation of channel quality.

FIG. 7 illustrates the relationship between the “forgetting function” λand measurement errors in the channel quality.

FIG. 8 illustrates the relationship between P_(diff-max) and channelquality.

FIG. 9 illustrates the relationship between the power step size Δp andthe soft value of the power order command.

FIG. 10 illustrates the relationship between the “forgetting function” λand the soft value of the power order command.

The invention will now be described in more detail with reference topreferred exemplifying embodiments thereof and also with reference tothe accompanying drawings.

DETAILED DESCRIPTION

A preferred embodiment of the method according to the invention will bedescribed by using a CDMA-system as an example without being restrictedto that. For example, traditional systems like TDMA (Time DivisionMultiple Access) and TDD (Time Division Duplex) also have power controlmethods that should benefit from this inventive idea. In e.g. TDMA,power control commands could be sent every burst creating an in-bandsignalling channel for transmitting the power order commands on. Thefollowing is also assuming power control on the reverse link. Theproposed algorithm may, as is easily understood by a person skilled inthe art, also be realised for power control on the forward link. In thatcase it is the mobile terminal that delta modulates the radio channelquality instead of the base station delta modulating the received power.

FIG. 1 shows a schematic picture of a CDMA cellular radio communicationsystem to which the invention preferably can be applied. The systemcomprises several base stations 130 communicating with mobile terminals100-120 located within its area of coverage. From each mobile terminal100-120 to its respective base station 130 is a reverse link 140established and from the base station 130 to its respective mobileterminals 100-120 is a forward link 150 established. A closed loop powercontrol is achieved by transmitting power commands on the forward link150 or on the reverse link 140 ordering the mobile terminal 100-120 orthe base station 130 to raise or lower its output power.

In the following a closed loop reverse link power control method,according to the invention, is described while referring to FIGS. 2-10.

FIG. 2 shows a flowchart of the power control method according to theinvention. In the first step 200 the quality Q of signals received fromthe mobile terminal on the reverse link is determined in the basestation. This may for example be done by a mapping of values from theRake-receiver to a C/I (Carrier-to-Interference) value in dB. Theresulting channel quality Q is then used to calculate in the basestation the desired output power {tilde over (P)}_(n) 210 from themobile terminal through an iterative formula. The reverse link channelquality Q_(n−1) and the estimated previously used output power{circumflex over (P)}_(n−1) from the mobile terminal are used as inputto the iterative formula according to the relationship:

{tilde over (P)} _(n)=α−β·(Q _(n−1) −{overscore (P)} _(n−1))(dB)

In this iterative formula the constant a sets an average power level inthe system and β is a function that determines the amount of feedback ofestimated previously used output power values {circumflex over(P)}_(n−1) from the mobile terminal in relation to channel qualityQ_(n−1). Thus, the desired output power {tilde over (P)}_(n) iscalculated on basis of the actual quality Q_(n−1) on the channel, andnot compared to an SNR threshold as in prior art solutions. In apreferred embodiment, α and β have the values α=−5 dB and β=0,7, butother values are, of course, possible.

After the desired output power {tilde over (P)}_(n) from the mobileterminal has been calculated in the base station it is delta-modulated220, i.e. it is determined whether the desired output power {tilde over(P)}_(n) is lower or higher compared to the estimated previously usedoutput power {circumflex over (P)}_(n−1) from the mobile terminalaccording to the relationship:

TPC=sign({tilde over (P)} _(n) −{circumflex over (P)} _(n−1))

The TPC (Transmit Power Command) can take two values, one valueindicating that the mobile terminal should raise its output power andone value indicating that the mobile terminal should lower its outputpower. A power order command ordering the mobile station, depending onthe TPC, to either raise or lower its output power, is then transmittedfrom the base station 230, to the mobile terminal.

To avoid control loop delays the power order command is transmittedwithout channel encoding and outside any interleaving and may thus besubject to bit errors. The base station therefore cannot know whatdirection the mobile terminal will take and the output power from themobile terminal is thus estimated according to the relationship:

{circumflex over (P)} _(n) =Δp·TPC+λ·{circumflex over (P)}_(n−1)+(1−λ)·P ₀(dB)

In this iterative formula λ is a “forgetting function” reducing theinfluence of the estimated previously used output power {circumflex over(P)}_(n−1) from the mobile terminal and Δp is an adjustable power stepsize function. The “forgetting function” λ is used in order to avoidpower drift between the desired output power {tilde over (P)}_(n) andthe actually used output power P_(n)·P₀ is a constant power value,assigned based on system conditions.

The mobile terminal, on the other hand, does not know the actual valueof the TPC. Instead an estimate {overscore (TPC)} of the transmitted TPCvalue is used in the mobile terminal. To make the mobile terminal andthe base station utilise the same values of Δp and λ a valueq_({overscore (TPC)}) may be returned to the base station which is usedto calculate Δp and λ. The values of Δp and λ are then transmitted tothe mobile terminal. When calculating the new output power 240 the valueof {overscore (TPC)} together with the actual value of P_(n−1) is used.The calculation is performed according to the relationship:

 P _(n) =ΔP·{overscore (TPC)}+λ·P _(n−1)+(1−λ)·P ₀(dB)

where the values of Δp and λ, as said, are provided by the base stationso that the base station and the mobile terminal utilise the same valuesin the respective calculation.

Referring now to FIG. 3 a summary is made regarding where the differentcalculations are performed. In the base station is first the qualityQ_(n−1) of the received signal from the mobile terminal determined. Thevalue Q_(n−1) is then used to calculate the desired output power {tildeover (P)}_(n)=α−β·(Q_(n−1)−{circumflex over (P)}_(n−1)) from the mobileterminal. A power order command TPC is calculated in the base stationand transmitted to the mobile terminal. The mobile terminal receives anestimate {overscore (TPC)} of TPC and returns a valueq_({overscore (TPC)}) that is used in the base station to calculate Δpand λ. The new output power is calculated in the mobile terminalaccording to P_(n)=Δp·{overscore (TPC)}+λ·P_(n−1)+(1−λ)·P₀. In the basestation signals are received with the new output power P_(n) and thequality Q_(n) is determined. The base station also calculates anestimate {circumflex over (P)}_(n)=Δp·TPC+λ·P_(n−1)+(1−λ)·P₀ of thepower used in the mobile terminal.

Usually power control methods in CDMA systems try to achieve a constantC/I in all possible situations. Hence, for links with a very small pathloss the output power is very small and when the path loss is high theoutput power is high. The iterative formula {tilde over(P)}_(n)=α−β·(Q_(n−1)−{circumflex over (P)}_(n−1)) with a value of βbetween 0 and 1 implies however, that mobile terminals experiencing asmall path loss are allowed to transmit with a larger output power thancalculated in the open loop control and mobile terminals experiencing alarge path loss have to transmit with a smaller power than calculated inthe open loop power control. This increases the robustness for linksexperiencing a small path loss towards sudden interference peaks andlimits the output power for the largest system interferers. Thus, thetotal system interference is reduced.

In a preferred embodiment of the invention the time correlation ofchannel quality sets the average level of the power step size Δp, i.e.determines the size of the power step Δp. In FIG. 4 is Δp shown as afunction of time correlation of the channel quality. At low timecorrelation of the estimated channel quality, i.e. when there are largedifferences in estimated quality between two time instances on thechannel, the average power step size Δp should be small to reducefluctuations in output power. When the time correlation of estimatedchannel quality is high, i.e. when the estimated channel quality issimilar between two time instances, the average power step size Δpshould be larger to react faster to changes in the channel. However,when the correlation is very high the output power should already bevery close to the desired output power and thus the average power stepsize Δp should be made small again. For example, frequency hoppingsystems have lower time correlation of channel quality compared tosystems without frequency hopping.

The changes in time correlation of channel quality could also depend onmeasurement errors. Referring to FIG. 5 Δp is shown as a function of thecorrelation of the measurement errors ρ_(merr) and the variations inmeasurement errors σ_(merr). When the correlation of measurement errorsρ_(merr) is high and the variation in measurement errors σ_(merr) is lowΔp should be made large, since the channel behaves similar between twotime instances. When the variation in measurement errors σ_(merr) growsand ρ_(merr) is still high Δp should be larger, since the channelvariations are larger. On the other hand, when the correlation ofmeasurement errors ρ_(merr) is low and the variation in measurementerrors σ_(merr) is low Δp should be made small, since the knowledge ofthe next state of the channel is unceertain. When the variation inmeasurement errors σ_(merr) grows larger and ρ_(merr) is still low Δpshould be made a little bit larger but still small since the behaviourof the channel is uncertain. This leads to lower variations in outputpower and thus to less interference.

FIG. 6 shows the “forgetting function” λ as a function of timecorrelation of the channel quality. At low time correlation of thechannel quality, i.e. when there are large differences in qualitybetween two time instances on the channel, the forgetting factor λshould also be low, i.e. reducing the impact of the previously usedoutput power to reduce fluctuations in output power. When the timecorrelation of channel quality is high, i.e. when the channel quality issimilar between two time instances, the “forgetting function” λ shouldbe large so that old values have greater impact on the system and itthus can react faster to quality changes in the channel.

Measurement errors in the channel quality affect also the “forgettingfunction” λ. In FIG. 7 is the “forgetting function” λ shown as afunction of correlation of the measurement errors ρ_(merr) and thevariations in measurement errors σ_(merr). When the correlation ofmeasurement errors ρ_(merr) is high and the variation in measurementerrors σ_(merr) is low λ should be made large, since the channel behavessimilar over time. When the variation in measurement errors σ_(merr)grows and ρ_(merr) is still large, λ is made a little bit smaller butshould still be large, since the channel variations are larger. On theother hand, when the correlation of measurement errors ρ_(merr) is lowand the variation in measurement errors σ_(merr) is low λ should be madesmall, since the knowledge of the next state of the channel isuncertain. When the variation in measurement errors σ_(merr) growslarger and ρ_(merr) is still low, λ should be made a little bit largerbut should still be small since the behaviour of the channel isuncertain. This leads to lower variations in output power and thus toless interference.

In FIG. 8 is yet another embodiment of the invention shown where thepower step size Δp is made dependent on a long-term mean value P_(mean)of the desired output power {tilde over (P)}_(n). If thedelta-modulation results in a desired output power {tilde over (P)}_(n)that differs too much from the long-term mean value P_(mean), a powerchange in that direction is ignored. The difference between thelong-term mean value P_(mean) and the desired output power {tilde over(P)}_(n) is defined as:

P _(diff) ={tilde over (P)} _(n) −P _(mean)(dB)

P_(diff) should preferably be made a function of current channel qualityQ and be different depending on if P_(diff) is positive or negative.P_(diff) should also be bound by a maximum value P_(diff-max) when thedifference is positive and a value P_(diff-max) when the difference isnegative. In FIG. 8 an allowed area is represented between the twocurves representing P_(diff-max) and P_(diff-max). When the channelquality Q is low, a larger difference in P_(diff) should be accepted tomanage deep dips in e.g. C/I, but values of P_(diff) outside the areabetween the two curves representing P_(diff-max) and P_(diff-max) shouldbe ignored, i.e. a power change that results in a value outside the twocurves are not performed. This results in reduced fluctuations in usedoutput power.

FIG. 9 shows the average power step size Δp as a function of the softvalue of the power order command. If the soft value indicates a largeprobability of bit error the risk of error in the power order is large.The power step size should therefore be small to reduce fluctuations inused output power. If the soft value instead indicates a low probabilityof bit error the power step size can be larger, thus reacting faster tochanges in the channel.

FIG. 10 shows the “forgetting function” λ as a function of the softvalue of the power order command. If the soft value of the power ordercommand indicates a large probability of bit error the “forgettingfunction” λ should be low, i.e. reducing the influence of the previouslyused output power value to reduce fluctuations in output power. When thesoft value instead indicates a low probability of bit error the“forgetting function” λ should be high giving old power values higherimpact so that the system can react faster to quality changes in thechannel.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the scope of the invention, and all such modifications aswould be obvious to one skilled in the art are intended to be includedwithin the scope of the following claims.

What is claimed is:
 1. A method of controlling transmitting power in acellular radio communication system comprising one or severaltransmitters each communicating with receivers located within itsrespective area of coverage the method characterised by the followingsteps determining in the receiver the quality (Q) of signals receivedfrom the transmitter; calculating in the receiver a desired output powerlevel ({tilde over (P)}_(n)) from the transmitter as a function ofestimated previously used output power ({circumflex over (P)}_(n−1))from the transmitter in relation to channel quality (Q_(n−1)) from thetransmitter to the receiver; delta-modulating in the receiver thedesired output power ({tilde over (P)}_(n)) by comparing it with theestimated previously used output power ({circumflex over (P)}n−1) fromthe transmitter to determine a power order command; transmitting thepower order command to the transmitter ordering it to either raise orlower its output power; and calculating in the transmitter its newoutput power dependent on the received power order command, a power stepsize function (Δp), previously used output power (P_(n−1)) and aforgetting function (λ).
 2. The method of claim 1 further characterisedin making the average power step size function (Δp) a function of timecorrelation of the channel quality (Q) from the transmitter to thereceiver.
 3. The method of claim 1 further characterised in determiningin the transmitter the reliability of the power order command; makingthe average power step size function (Δp) a function of the reliabilityof the received power order command.
 4. The method of claim 1 furthercharacterised in determining the difference P_(diff)={tilde over(P)}_(n)−P_(mean) where P_(mean) is a long term mean value of thedesired output power {tilde over (P)}_(n); and making the maximum ofP_(diff) a function of channel quality (Q) from the transmitter to thereceiver.
 5. The method of claim 1 further characterised in making theforgetting function (λ) a function of time correlation of the channelquality (Q) from the transmitter to the receiver.
 6. The method of claim1 further characterised in making the forgetting function (λ) a functionof the reliability of the received power order command.
 7. The method ofclaim 1 further characterised in calculating the desired output power({tilde over (P)}_(n)) in accordance with the relationship {tilde over(P)} _(n)=α−β·(Q _(n−1) −{circumflex over (P)} _(n−1))(dB) where α setsan average power level in the system and β is a function whichdetermines a feedback of estimated previously used output power({circumflex over (P)}n−1) from the receiver in relation to channelquality (Q_(n−1)) from the transmitter to the receiver.
 8. The method ofclaim 1 further characterised in delta-modulating the desired outputpower ({tilde over (P)}_(n)) in accordance with the relationshipTPC=sign({tilde over (P)} _(n) −{circumflex over (P)} _(n−1)) where{circumflex over (P)}_(n−1) is estimated previously used output powerfrom the transmitter.
 9. The method of claim 1 further characterised incalculating the new output power in accordance with the relationship P_(n) =Δp·{overscore (TPC)}+λ·P _(n−1)+(1−λ)·P ₀(dB) where {overscore(TPC)} is the received power order command and λ is a forgettingfunction reducing the influence of previously used output power P_(n−1)and Δp is an adaptable power step size function.
 10. The method of claim1 further characterised in determining the reliability of the powerorder command by checking the soft value of the power order command asit is received in the transmitter.